Effects Of Li Content On Formation Process Of Mao Coating On Mg-Li Alloys

For the Mg-Li alloy, different phase correspond to different content of Li in the alloys. Therefore, the content of Li do a important part for the film coating in the oxidation process. As far as this problem is concerned, the technique of MAO was used for the preperation of coatings on Mg-5% Li alloy, Mg-8% Li, Mg-11% Li and Mg-14% Li alloys. With regard to the result, the mechanism of the MAO coating formation was studied.X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDS) were applied for analyze the phase and elemental composition of the ceramic coatings. Scanning electron microscopy (SEM) was employed to observe the surface and section morphology. Ectric eddy current thickness gauge was used to calculate the thicknesses of the ceramic coatings. TR200 roughness was used to calculate film roughness. In addition, inductively coupled plasma mass spectrometry (ICP-MS) was applied to confirm the precipitation of lithium in the process of coating formation.Growth process of the ceramic coatings on four kinds of alloy were studied in a single system and the influential factors such as Voltage curve, coating thickness, coating roughness,surface morphology, surface elements, and the changes of Li+ concentration in solution. The probable formulas of MAO process on alloy with various Li content were deduced, and the effect of Li on formation of MAO coating was concluded. The MAO voltage would be drop as the content of Li increased in the alloy. The films were mainly consisted of MgO and Mg2SiO4 crystallized phases, but the growth time of Mg2SiO4 in the coating is different on the different alloys:α-phase +β-phase>β-phase>α-phase. Furthermore, the content of lithium ascended in the electrolyte throughout MAO reaction, and the precipitation of lithium: 14%>11%≈8%>5%. That notes there would be precipitated as more as the content of lithium increased in the alloy. Besides, there will be Li+ segregation in the surface of the alloy during the MAO process, and the crystal phase transition occurs:β-phase (Li₃Mg₇)→α-phase (Li0.92Mg₄.08). In addition, different additives do a different part in the prcocess of films coatingin MAO. In this research, the additives are NaOH and NaF. Similarities of the two additives are that increasing the conductivity,dissoluting the coating and having little effect on the reaction process. However the two have different influence in the growth rate of crystallized phases such as MgO and Mg₂SiO₄. Specificly, the growth rate is lower in Na₂SiO₃+NaF electrolyte. In this experiment, the presence of OH- would be increasing the growth rate of coating at lower concentrations, but to contrary, the presence of OH- would be dissoluting the coating at higher concentration; F- would be inhibiting the dissolution to promote the growth of the film because of the F- enrichment in the surface of substrate.